Text-Book of Embryology 2-11 (1919)

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A personal message from Dr Mark Hill (May 2020)  
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I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!

Kerr JG. Text-Book of Embryology II (1919) MacMillan and Co., London.

Textbook Chapters: 1 Formation of the Germ Layers | 2 Skin and Derivatives | 3 Alimentary Canal | 4 Coelomic Organs | 5 Skeleton | 6 Vascular | 7 Internal Body Features | 8 Adaptation to Environmental Conditions | 9 General Considerations | 10 Common Fowl | 11 Lower Vertebrates | Appendix

- Currently only early Draft Version of Text -

Historic Disclaimer - information about historic embryology pages 
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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Chapter XI Hints Regarding the Practical Study of the Embryology of the Various Types of Lower Vertebrates

Amphioxus

The interest and importance of Amphioxus to the student of Vertebrate morphology are due to the fact of its position near the base of the Vertebrate phylum. It is true that in its adult structure Amphvlomus is intensely specialized in correlation with its burrowing habit. Further, it is necessary to recognize that a burrowing like a pelagic mode of life, in which the environmental conditions are comparatively uniform, is likely to lead to a kind of fixing of the organization which will be fatal to its adaptability to new sets of conditions and consequently to its capacity for evolving along new lines. We must therefore regard it as improbable that the Vertebrata passed through an ancestral condition of specialization for a burrowing habit and the specialized features of the later stages of the life history of Amphioxus cease on that account to have a phylogenetic interest. The main interest to the Vertebrate morphologist lies therefore in the earlier stages before the specialization of the adult has developed——in such features as segmentation, gastrulation and the origin of the main systems of organs. And the interest of these stages is heightened by the fact that food yolk-— that potent disturbing factor—is present to a far smaller extent in the egg of Ampiaxioxus than in that of any other of the lower Vertebrates.

Unfortunately the known localities in which fresh ernbryological material of Amphioxus can be obtained in abundance are still few, and in most laboratories recourse must be had to preserved material purchased from supply stations such as the Naples aquarium.

The best locality so far known for obtaining developmental stages of Amphiomus is the pantano or shallow lagoon at Faro near Messina. Here the spawning takes place each evening, when conditions are favourable, during the summer months from April to July. The eggs pass to the exterior through the atriopore. If in a dish on board a boat the eggs are liable by its movements. to become distributed through the water and they are then apt to become drawn by the inspiratory current in amongst the buccal cirri. When the Amphibians becomes inconvenienced by such entangled eggs amongst the cirri it is able suddenly to reverse the respiratory current so as to clear them away, and in this Way there is produced a misleading appearance as if the eggs were being laid through the mouth. The first meiotic division has been completed before oviposition while the second is in the spindle stage at this period. Fertilization probably takes place immediately, spermatozoa being disseminated through the water.

It is best (Cerfontaine, 1906-7) to bring the adults into the laboratory and wait until they spawn which operation may be considerably delayed. To a dish of pure ‘sea-water is added a little sea-water containing sperm then the eggs, collected with a pipette as soon as extruded, are added.

Batches of eggs are fixed periodically, preferably in strong Flemming’s solution or Hermann’s solution. After dehydration they are placed in a mixture of 2 parts clove oil and 1 part collodion in which they may be kept indefinitely. For examination whole the egg or embryo is placed on a slide or|coverslip in a drop of the clove-oil-collodion. After the specimen has been arranged in .the desired position by means of needles a drop of chloroform is applied in order to cause the collodion to solidify. The whole is then cleared with cedar oil and mounted in canada balsam. For the preparation of sections the procedure is similar, only in this case the slide or coverslip should be coated with paraffin as a preliminary to allow the collodion block to become detached, and the latter should be embedded in paraffin.

Petromyzon

The various species of Lamprey make their way up streams to suitable gravelly spots for spawning in the spring or early summer (April, May, in the northern hemisphere). Material for emhryological study is best got by “stripping ” the ripe males and females 7}.e. by passing the hand back along the body with gentle

pressure so as to force out the eggs or sperm. The gametes from the male and female are collected separately in two small dishes: they are then mixed together, stirred gently with a feather, and water added. This “ dry ” method gives a smaller proportion of unfertilized eggs than when the eggs are received from the fish directly into water (Herfort, 1901). As fixing agent the ordinary corrosive sublimate and acetic acid is quite satisfactory.

Myxinoids

The only Myxinoid eggs that have been obtained in any numbers are those of Bdellnstoma which are dredged near Monterey, California, on shelly and gravelly bottom at a mean depth of about 12 fathoms (Bashford Dean, 1899). Much still remains to be done in working out the details of their development but it is clear that this is of a highly peculiar and specialized type.

ELASMOBRANCHI1.

The eggs are fertilized in the upper part of the oviduct. They may traverse the oviduct comparatively rapidly and be laid as in Birds at an early stage of development [0’h'£mae'ra, Scylliidae, Castration, Rain] or they may remain in the oviduct for a prolonged



FIG. 247.—-Blastoderm of Torpedo with meulnllary folds x 18). (After Ziegler, 1892.)

A, stage four (b'c.'unmon, 1911); B, stage six; 0, stage ten. The rounded projection near the anterior edge of the blastoder-m is the bulging I‘00f of the so-glm-IlL:1l.inlI (::|\'il.,\-'. In (I the bl()0«‘l-islamls form :1. row of (‘HlI.\‘pl(.'llIJll.*~‘. I-h-\'.'tt-inns of the :«'11rl':-we of the l)l:lh'lA)ll(‘I‘lll p:u':alls-I to its 4*Il;_{4-,


period and the young born in an advanced stage [Notidam/us, Mustalus, Galeus, Uarcharias, Zygaena, Lamna, Alopias, Uetorlzxinus, Acumtlmlas, Scymnus, Squatina, Torpedo, Trygonidae,Myliobatidae]. Amongst the viviparous Elasmobranchs preserved developmental stages of Torpedo (Fig. 247) may be obtained from Naples, and of Acanthias from various marine laboratories.


Amongst the oviparous forms certain species of Skate (Rania) are used as food-fishes and their eggs can frequently be obtained in quantity at trawling centres. In such cases arrangements can be made with local fish-dealers to send on by post the “ skate-purses ” taken from the oviducts when the fish are cut up.‘ The eggs of the different species differ in size and in the characters of the shellshape, colour, degree of translucency (Williamson, 1913). Of the European species B. batvls is the most convenient species to use; the normal period of spawning is from December to April but the retarding effect of the low temperature i.s so great that December eggs are practically overtaken in their development by the April eggs. The complete period of development is roughly 20 months, most of the eggs hatching about August.


The eggs should be posted in damp seaweed. On arrival the soft sticky marginal zone of the shell, which separates off except at one end and serves to anchor the egg to the sea-bottom, is removed, and the date is marked in ink with a wooden style upon the flat portion of shell between the two horns.


1 Jamieson observed out of many thousands of eggs only one case of the inclusion of two eggs within a common shell.


For hatching boxes it is convenient to take ordinary fish boxes freely perforated with anger holes, provided with a cross partition in the centre, and pitched inside and out to discourage the growth of seaweeds. The hatching boxes are moored afloat in pure sea-water within a breakwater or other shelter. About 20 eggs are placed in each compartment.


On alternate days the boxes are drawn a few times backwards and forwards through the water to dislodge any sediment that may have accumulated. Once a week they are hauled’out of the water and each egg-shell tested by rubbing the finger over its surface. If a slippery mucus-like layer has developed on its surface the egg is useless and should be got rid of.


When the egg has reached the desired period of development it is removed from the Water, placed in a horizontal position with the more strongly convex side below and opened by carefully removing the greater part of the less convex side of the shell. The isolated piece of shell must be lifted off very carefully as the albumen is very adhesive and the vitelline membrane extremely delicate.


In the early stages the embryo is almost invisible in the fresh state so the egg, still held carefully in a horizontal position, is gently submerged in fixing fluid. The blastoderm then comes into view and after a short time may be excised and floated into a watch-glass to complete fixation and the subsequent processes.


In later stages (Fig. 248) where the body of the embryo is constricted off from the yolk—sac, it is narcotized by submersion in sea-water containing alcohol and then the yolk-stalk is ligatured with thread and the embryo excised for further treatment.


Embryological material of the Sharks is to be preferred to that of the Skates or Rays on account of their less specialized character but unfortunately it is more diflicult to obtain in quantity. Small sharks of the genus Scyllelum and allied genera occur commonly round the shores of the various continents and their eggs may be found attached to seaweed at extreme low tides.


On the British coasts a well-known spawning ground for Sag/llium canicula exists at Careg Dion about 2% miles from Beaumaris on the Anglesea side of the Menai Straits in between 3 and 4 fathoms of water and in spots not exposed to strong tidal currents} The eggs are deposited usually in the morning, the shorter stouter pair of filaments which issue first from the cloacal opening being trailed about /amongst tufts of the seaweed Halidrys siliquosa until they become entangled when the fish swims round so as to wind the elastic filaments firmly amongst the seaweed. The eggs can only be obtained at very low and specially favourable spring tides and as White finds at one time embryos of all stages of development it would appear that oviposition is not limited to any definite season. ~ Sag/llwlum not infrequently deposits its eggs in aquaria and at the Berlin Aquarium it has been observed that pairs of eggs were deposited at intervals of about ten days. The methods of technique mentioned in connexion with the Skate are also applicable to the eggs of Scyllium.


1 For the details in regard to this locality I have to thank Professor Philip J. White of Bangor.


It should not be forgotten that, as mentioned earlier in this volume, one of the greatest desiderata in Vertebrate embryology is an oviparous shark with eggs of small size.


FIG. 248.—Raia batis, embryos.

at, atrial portion of heart; E, eye; c, conus ; f.g, foregut; H, heart; l, lens; Zi, liver; ot, otocyst; pin, pineal organ ; rh, thin roof of fourth ventricle; v.c.I, etc., visceral clefts; y.s, yolk-stalk; V, VII, VIII, cranial nerves.

Teleostomi

The most archaic and therefore the morphologically most important surviving member of this group is Polypterus and strenuous efforts have been made to obtain developmental material. Harrington lost his life on an expedition to the Nile with this object. Budgett made two expeditions to the Gambia, one to. Nigeria and the Nile, and a fourth to the Niger Delta. with the same object in view. The three first expeditions were fruitless but on the fourth he was fortunate enough to obtain ripe males and females and to accomplish fertilization of a number of eggs. Unhappily Budgett did not live to work out this precious material, falling a victim to blackwater fever soon after his return to England. The Budgett material has been investigated (Graham Kerr, 1907) but further material is urgently needed to work out much of the detail.


On the Gambia and on the Upper Nile Budgett found females with eggs in the oviducts during July and August; in the Niger Delta during August and September. During these periods he found that at any one time only a small proportion of males had active motile spermatozoa in their urinogenital sinuses so that it looks as if the actual breeding season of each individual male were very short. The fertilizations which were successful were effected with teasedup testis, the tubules being much distended and the sperm clear instead of opaque as it frequently is. In some cases Budgett found that eggs from the splanchnocoele gave a larger percentage of successes than those from the oviduct.


The fertilized eggs adhered strongly to the bottom of the dish and this supports the statements made by the natives that in nature the eggs are attached to sticks and stems ol' plants under the water.

Nothing is known regarding the development of the other surviving Orossopterygian-—Oalamichthys.

Of the Actinopterygian ganoids, whose haunts are more accessible

and less unhealthy than those of I’ol_2/pterus, the development has’

been worked out more or less completely in the case of each of the main types———the Sturgeon (Acipenser), the Garpike (Lepidosteus), and the Bowfin or Dogfish (Amia).

At the large fishery stations such as those on the Elbe or Delaware Rivers ripe Sturgeons are caught during a brief season on their way into the river to spawn. The eggs and spermatozoa may be obtained by “ stripping ” the fish 73.6. by firm pressure passed backwards along the sides of the body, or by opening the fish. The eggs are immediately placed in a dish and a little of the sperm mixed with a small volume of Water is poured over the eggs, the whole being stirred gently for about ten minutes. They are then distributed in a single layer over the bottom of a submerged shallow tray made with coarse mosquito netting to which the eggs adhere firmly within twenty minutes. ‘The trays are then placed in wooden hatching boxes with gauze ends and moored in the river so that they are traversed by a constant current. The dark-coloured somewhat tadpole-like larvae hatch out in from three to six days.

Lepidosteus (Dean, 1895) breeds at Black Lake, N .Y., normally between the middle of May and the middle of June, the eggs being fertilized at the moment of spawning and being distributed over the bottom in shallow water, adhering firmly to stones and other solid objects. For laboratory purposes it is best to employ artificial fertilization as in the caseof the Sturgeon. T Amia (Dean, 1896) spawns at Black Lake during the latter half of April or May. The eggs are deposited on a compact site over which the vegetation is pressed aside so as to form a clear space with about a foot of water over it. The eggs, fertilized at the moment of laying, adhere to roots or other portions of the water—plants. The rate of development as in other cases varies greatly with the


F10. 249.—Stages in the development of Symbmnchus. (After Taylor-, 1914.) our, optic rudiment; .I’.F, pectoral nu rudiment.

temperature and from four days to fourteen have been observed to elapse between the deposition of the eggs and their hatching.‘

Of Teleostei (Figs. 249 and 250) by far the most convenient for systematic laboratory work are the Salmon (Salmo salar) and the Trout (S. famlo), eggs of which can be obtained in quantity from the various hatcheries. The eggs obtained by “stripping” are fertilized artificially and may then be sent by post packed in damp moss. Small hatching boxes suitable for laboratory use can also be purchased?

The eggs and larvae of marine Teleosts are often obtained in great numbers in the tow-net


  • 1 Excellent developmental material of Lepidosteus and Anita may be obtained from the Woods Hole Laboratory or from Mr. J. C. Stephenson, Washington University, St. Louis. 9 E.g. from the Snlway Fisheries Co., Dumfries, Scotland. but these are not so convenient for investigation on account of their reduced size. As there is little doubt that the 'l‘eleostei have been evolved out of ancestral forms with large eggs investigations are particularly desirable on those teleosts, mostly freshwater forms inhabiting warm climates, in which the large size of the egg has been retained. There is an important field for investigation in the embryology of tropical freshwater fishes. Of individual families the Siluridae, Characinidae and Gymnotidae call especially for investigation.

Dipnoi

The Lungfishes form a group of much importance to the Vertebrate morphologist on account of, on the one hand, their great antiquity and the retention of many archaic features in their organization and, on the other hand, of the fact that they present to us foreshadowings of various features which become prominent characteristics in the tetrapoda or terrestrial animals. A knowledge of their embryology consequently became one of the great desiderata of Vertebrate Embryology. The first

- Fm. 250.——-Bla.stmlei‘1ns anal mubryos of Trout dlscovered of the three (Salmo fa/riu). (After Kopscll, 1898.)

surviving representatives of the gI‘0llp—-L6}9?«d0- y, oxpo.~ae(l su1'fa.ee of yolk.

Iv), eye; at, otocyst; p.f, pect.0r:ll Mn; -rh, rhombeneephalou; 566 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.

s7?rem—remained unknown so far as its development was concerned until 1896 when Graham Kerr succeeded in obtaining abundant embryological material in the Gran Chaco of South America.

The developmental stages of Protopterus, the next representative of the group to become known to science, were first obtained on the Gambia River by Budgett who had taken part in the Lepidosiren expedition a few years earlier. Ceratodus, the last of the surviving genera to become known in the adult condition, was the first to be made known embryologically by Caldwell and Semon as already mentioned (p. 435).

The Lung-fishes like other animals living under similar conditions breed at the commencement of the rainy season (Protoptems, Gambia, August; Lepidosiren, Ohaco, November but incidence of rainy season irregular and may be de1ayed—till e.g. J une-—or omitted altogether; Oeratodus, September to December). In the case of Oemtodus the eggs are scattered loosely about amongst the water plants, while in Protoptems and Lepidosiren they are deposited in a special burrow at the bottom of the swamp where they are guarded by the male parent.

¢ Dipnoans live well in captivity and there is little doubt that it will be found easy to induce them to breed by using similar methods to those described under the heading Amphibia. It is particularly desirable that this should be done in the case of Lepidosiren on account of the large size of its histological elements which make it a peculiarly suitable type for the investigation of various problems of histogenesis.

The eggs of Dipnoi, especially of Lepidosiren, are of large size and this makes it especially advisable to use celloidin in addition to paraffin methods of embedding. When paraflin is used it is necessary to remove the egg envelope by slitting it up with fine scissors, care being taken to keep the point of the scissors close to the envelope so as to avoid injury to the surface of the egg.

Corrosive sublimate and acetic acid is a good stock fixing agent. For stages before hatching 107 formalin is convenient.

Amphibia

The most easily obtained embryological material is that of the common Frogs of the genus Roma the masses of spawn of which are familiar objects in pools during the early weeks of spring in temperate climates. The exact time differs with climate and also with species, some species such as R. esculenta in Europe and R. catesbiana in North America lagging several weeks behind the others. The spawn, fertilized as deposited in the early morning, may conveniently be kept during its development in earthenware pans. The water should be left stagnant and unchanged during the period prior to hatching as under these circumstances the spawn is less liable to be attacked by fungus but the hatched larvae should be at once transferred to clean water.

Investigations are greatly needed on the embryology of Anura outside the genus Rana (of. Figs. 251, 252, 253 and 254). The different genera and species differ greatly in the size of the egg XI DIPNOI, AMPHIBIA 567

and its richness in yolk and there is no group of Vertebrates which ofi"ers anything like the same facilities for studying the influence of yolk upon the course of development. Further it will be only after greatly extended studies on different species that we shall be in a position to have a really com- prehensive idea of typical Anuran development.

Many tropical species of Frogs and Toads fire to be


Fig. 251. String of eggs of unknown Frog from the Gambia. Obtalned ahve from Individual variations in the rate of developnu.-nt are indicated animal dealers and hy the varying size of the yolk-plug.

in these it may be taken as a general rule that breeding takes place at-the commencement of the rainy season, or in other words when environmental conditions become favourable after a prolonged period during which they have been unfavourable. By bear ing this principle in mind such tropical T i amphibians may usually be induced to breed in captivity. Bles in his excellent account of the life-history of Xenopus (1905) describes a method which will be found to be of general use. The pair of animals were kept in a Budgett tropical aquarium consisting of a glass bell-jar 20 inches in diameter dipping into a galvanized iron water-tank heated by a small Bunsen burner and oxygenated by plants of Vallisnemla. During summer the temperature of the water in the belljar was kept at about 25° C. The water was not changed. The frogs were fed daily with small earthworms or thin strips of raw calf’s liver until they would eat no more. In December the temperature was allowed to fall to 15°-16° during the day and as low as 5"-8° during the night. As the temperature rose With the °;lfiI1:ucca'1wity_M bmmpm_ onset of spring the frogs became more N’ mes, n1c.s'odcrnlsnigincnts. 4, actives Wilkmg “P out of the lethargic condition induced by the winter's cold. Breeding was induced by simulating the natural conditions of the rainy season. The temperature was raised to about 22° 0. Each morning and evening about two gallons of the water was drawn off, allowed to cool for twelve hours and then returned to the aquarium in the form of a fountain of spray from the upturned end of a glass siphon drawn out to a fine point so as to produce the effect of a shower of rain. Within a week or two breeding took place.


The chief difiiculty in the way of cutting sections of Frog’s eggs is due to the presence of the jelly-like envelope. This may be got rid of by prolonged soaking, six months or more, in -5% formalin (Ogushi, 1908), or by fixing in Zenker’s fluid and leaving the efgs 111 this fluid renewing it after 2 to 3 days and continuing the treatment for 8 to 14 days or longer, shaking gently so as to remove the envelopes (Kallius, 1908). '



FIG. 253.-—Stages in the development of ]’l:.y/llunwdusa /1.3/po¢:h¢mal7"£alz's. E, eye; e.g, external gill; op,opercu1um; oz, otocyst.


For cutting sections paraflin is commonly used but it should be supplemented by celloidin e.g. the clove-oil method mentioned under Ampimloxus.

In the Urodeles the eggs are commonly laid singly in water and attached to water plants (Triton) or other solid objects such as logs or stones (Proteus, Necturus). In Oryptabranchus and Amphiuma they form a beaded string, adjacent envelopes being connected together by a narrow isthmus.

Fertilization is rarely external (0'ryptob'ranchus——Smith, 1912). In the Newts the female takes up a spermatophore into the cloaca.

Such internal fertilization leads up to the condition in the Salamanders where fertilization takes place in the upper part of the oviduct and the developing embryo is retained for a less or more prolonged period within the body of the parent. In Salamcmdm mooculosa larvae about an inch in length are born in May resulting from fertilization during the preceding summer. '

As in the Anura wide differences exist in the richness of yolk and consequent size of the egg—the latter varying from under 2 mm. in the Newts to 6 mm. (Necturus) or 7 mm. in diameter (Org/ptobranchus japom'cus): so that here again though not to the same extent as in the Anura there is an excellent field for investiga c tion into the influence of yolk upon developmental processes. The eggs of Urodeles are commonly collected under natural conditions and kept in earthenware dishes. Or the adults just about to breed may be brought into the laboratory and allowed to deposit their eggs in a suitable aquarium;


Fig. 254. - Tadpo1e of unknown Frog from Tropical Africa.

A, side view; B, ventral view. inc, huccal cavity; c.o, ('(‘.lllf‘.I|l.r organ ; rz, anus; E, eye; e.g, external gill; u/,/', olfactory organ; up, operculum.


The Urodela form one of the relatively primitive groups of Vertebrates and their embryology‘ deserves much greater attention than it has hitherto received. Most of the older literature deals with special details i.n the development of the Newts but comprehensive monographs, including “normal plates” on the development of such genera as Proteus, Siren and Amphiuma are much wanted. A general account of the development of the American species of Uryptobranohus has been given by Smith (1912), while the Japanese species has been dealt with by Ishikawa (1918), De Bussy (1915) and Dan. de Lange, Jr. (1916). Of Necturus normal plates with accompanying tables have been worked out by Eycleshymer and Wilson (1910).


The Gymnophiona—-—though an aberrant group of Amphibians highly specialized for a burrowing existence—are of much embryological interest and have provided the material for work of great morphological importance, such as that of Brauer upon the excretory organs. A general account of the development of Icltthyophis will be found in Sarasin (1887-90) and of Hypogeophis in Brauer (1897).

The eggs, fertilized internally, are normally deposited in the soil and the embryologist has, as a rule, to depend upon such scanty material as can be obtained by digging in the damp soil of localities where Grymnophiona are abundant. Ty/phlonectes in South America and _De7~mophz's in West Africa are viviparous.

Of the group in general it may be said that a comprehensive monograph on the development of each genus beyond Ichthyoplmls and Hypogeoph/is is a great desidcratum.


As standard fixing agents for Amphibia corrosive sublimate and acetic acid, and for the later larval stages strong F lemming’s solution, may be used. For the early stages (segmentation and gastrulation) quite good results are obtainable from eggs that have been preserved alive in 10,°/O formalin: in this case it is well to treat the egg before dehydration for an hour or two with corrosive sublimate solution as without this precaution the formalin-preserved eggs are diflicult to stain well. When any other fixing agent than formalin is used it is necessary, as a preliminary, to remove the egg envelopes. In the case of the larger eggs of the Urodela and Gymnophiona this can be accomplished with the aid of fine scissors and forceps.

Reptilia

For gaining practical knowledge of Reptilian development the student will find the group Chelonia most convenient as it is possible to obtain 1 excellently preserved series of developmental stages of Terrapins (Ohrysemg/s) and Snapping Turtles (0helg/dm). In particular localities especially in warm climates he may have opportunities of obtaining the eggs of Lizards, Snakes or Crocodilians. In all cases the same technique may be used as in the case of the Fowl. ' AVEs.—The Birds, although showing conspicuous differences in external appearance and in minute details of structure, form a very compact evolutionary group and there is little likelihood of important differences in principle existing in their development. Interesting differences in detail however are to be found—such as the presence or absence of neurenteric canals. Groups which there is any reason to suspect of being particularly archaic——such as Divers, Grrebes, Penguins-—-are worthy of careful scrutiny for possible persistence of Reptilian features.


Literature

B103. Trans. Roy. Soc. Edin., xli, 1905.

Brauer. Zool. Jahrbiicher (Anat.), x, 1897. do Bussy (do Lange), L. P. Eerste ontwikkelingsstadién van Megalobatrachcpos

Mamimus, Schlegel. Amsterdam, 1905.

Cerfontaine. Arch. de Biologie, xxii, 1906.

Dean, Bashford. Journ. Morph., xi, 1895.

1 E.g. from Mr. J. C. Stephenson,‘Washington University, St. Louis,'or The-Marine Biological Laboratory, Wood's Hole. XI -

Dean, Baahford. Quart. Journ. Micr. Sci., xxxviii, 1896.

Dean, Bashford. Kupifers Festschrift. J ena, 1899.

Eerfort. Arch. mikr. Anat., lvii, 1901.

Iahikawa. Mitt. Deutsch. Gesell. Natur- und Viilkerkunde Ostasiens, xi, 2, 1908. Kalliua. Anat. Anz., xxxiii, 1908.

Kerr, Graham. The Work of John Samuel Budgott. Cambridge, 1907.

Kopsch. Arch. mikr. Ana.t., Ii, 1898.

do Lange, Dan., Jr. Onderzoek. Z061. Lab. Groningon, iv, 1916.

Ogushi. Anat. Anz., xxxiji, 1908.

Sarasin, P. and H. Ergebnisse nnturwissenschaftlicher Forschungen auf Ceylon, ii. Wiesbaden, 1887-90.

Soammon. Keibels Normentafeln, xii. Jena, 1911.

Smith. Journ. Morph., xxiii, 1912.

Taylor. Quart. Journ. Micr. Sci., lix,\1914.

Williamson. Fisheries, Scotland, Sci. Imwst., 1912, i. 1913.

Ziegler, H. E. and F. Arch. mikr. Ana.t., xxxix, 1892.